Direct laser sintering of metal inks is a known technique for metallization of printed wiring. For example, U.S. Patent Application Publication 2008/0286488 describes a method of forming a conductive film based on depositing a non-conductive film on a surface of a substrate. The film contains a plurality of copper nanoparticles, and exposing at least a portion of the film to light makes the exposed portion conductive by photosintering or fusing the copper nanoparticles.
Kumpulainen et al. describe direct laser sintering techniques in “Low Temperature Nanoparticle Sintering with Continuous Wave and Pulse Lasers,” Optics &Laser Technology 43 (2011), pages 570-576. The authors relate to “printable electronics,” in which nanoparticle inks, printed on the surface of a substrate, contain additives, such as dispersing agents and carrier fluids, that provide good printing properties by changing the viscosity and separating the nanoparticles of the ink. In the sintering process, ink particles are heated to a certain, ink-specific temperature, and the carrier fluid and dispersing agents are evaporated from the ink. Additional heating after evaporation causes the nanoparticles to start to agglomerate. Laser sintering is said to enable short sintering times and selective sintering, making it possible for printed structures to contain fragile active components produced with other technologies. The paper describes tests done with two different types of laser: pulsed and continuous wave.
After the priority date of the present patent application, Theodorakos et al. described further laser sintering techniques in “Selective Laser Sintering of Ag Nanoparticles Ink for Applications in Flexible Electronics,” Applied Surface Science 336 (2015), pages 157-162. The authors investigate the potential of three different laser sources: continuous wave (CW) or pulsed nanosecond and picosecond lasers, operating at 532 and 1064 nm, as efficient tools for selective laser sintering of Ag nanoparticle ink layers on flexible substrates. Theoretical simulations indicate that picosecond laser pulses restrict the heat-affected zone to a few micrometers only around the irradiated regions of the ink layer. These predictions were confirmed experimentally.